Thermotropic, liquid-crystalline polymers are becoming increasingly important for plastics processing. These products are wholly or partly aromatic polycondensates, such as polyesters, polyesteramides, polyetheresters, polyestercarbonates, polyesteramidoimides and similar polymers. One characteristic of the thermoplastic processing of the above-mentioned polymers is that, when strain flows and shear flows act on the liquid-crystalline melt, its molecular chains can be oriented in a controlled manner. Thus, this effect includes the possibility of producing both products having isotropic mechanical properties and those having anisotropic mechanical properties, depending on the degree of orientation. The most frequently encountered case of anisotropic behavior in practice results from monoaxial deformation of the melt film while being forced out of a die, preferential orientation in the longitudinal direction of the molding, for example of a film web, being imposed on the molding. There are consequently generally outstanding strength properties in the longitudinal direction, which however are obtained at the expense of the property profile in the transverse direction. In the case of sheet-like structures, such as, for example, films or sheets, splicing is observed in the transverse direction when external mechanical forces act at right angles to the preferred orientation direction. A film or a sheet showing such behavior is only of limited use in practice.
DE-A-36 03 995 discloses a process for the production of shaped parts from thermotropic, liquid-crystalline plastics having a defined orientation of the LC polymer molecules in at least two preferred directions independent of one another, in which the thermoplastic melt stream is extruded through a die. At least temporarily while flowing through the die, the melt stream is subjected, in two or more predetermined directions, to strains for orienting the chain molecules, by means of forces determined by the shape of the bounding surfaces. The strain in the x direction is established in a predetermined direction by the set spacing of the bounding surfaces. The strain in the flow direction is fixed by predetermined functions which incorporate geometric parameters. The strain in the y direction is likewise fixed by a certain function with geometric parameters. In this known process, orientation of the molecular chains is effected inside a die, for example a slot die, by virtue of the fact that surfaces bounding the melt stream are appropriately shaped inside the die.
German Offenlegungsschrift DE-A-36 03 432 discloses an extruder head for the production of semifinished products from thermoplastics, having a flow channel, the flow channel being capable of being divided into at least two individual channels in a section by means of a displacement element arranged in the axial direction, and the individual channels being combined again immediately before the mouth of the flow channel. The displacement element has a plurality of guide means oriented essentially parallel to one another or radially in the direction of a common point of intersection and at an angle of, for example, 45.degree. to the direction of flow in the flow channel. The end sections of the displacement element taper toward one another. The guide means bounding one individual channel are oriented opposite the guide means bounding the other individual channel. In this extruder head, two films are produced from the plastified material emerging from the extruder and are subsequently combined. The melt stream introduced from the extruder into the extruder head is divided by the displacement element into at least two part streams which flow through separate channels. Because of the particular design and arrangement of the guide means on the displacement element, the part streams are deflected from the original direction, one part stream preferably being deflected in the opposite direction with respect to the other part stream.